Bootstrap Circuitry

Abstract

This invention generally relates to a bootstrap circuit for a switch mode power supply, a controller for a switch mode voltage converter, a switch mode flyback converter comprising the bootstrap circuit, a switch mode forward converter comprising the bootstrap circuit, and a method of bootstrapping a switch mode power converter. The bootstrap circuit comprises: a current bleed impedance (Rht 1 ) to bleed current from an input power supply (VH+); circuitry to deliver current from the input power supply (VH+) via the current bleed impedance (Rht 1 ) to the base of a power switch (Q 1 ) such that the power switch (Q 1 ) is operable to amplify the current delivered from the internal power supply; a passive circuit (Dst) to provide the amplified current to a reservoir capacitor (Cdd); and the passive circuit element (Dst) further to substantially block reverse current flow from the supply input (Vdd) to the emitter of the power switch (Q 1 ).

1 . A method of bootstrapping a switch mode power converter (SMPC),
 the SMPC having:
 an input to receive power for said power converter; 
 an internal power supply derived from said power received at said input; 
 an output to provide a DC output voltage; 
 a power switch; 
 a controller to control said power switch, said controller having a threshold operating voltage; 
 a transformer having a primary winding and a secondary winding coupled between said input and said output; 
 said secondary winding being coupled to provide power for said DC output; 
 said primary winding being coupled in series with said power switch and coupled to receive power derived from said input to receive power; 
 an auxiliary power supply configured to provide a power supply to said controller; 
 the SMPC further comprising a controller power supply capacitor to store charge derived from said auxiliary power supply during operation of said SMPC for providing said power supply to said controller, 
   the method comprising:
 bleeding current from said internal power supply into the input of a current amplifier; and 
 charging said controller power supply capacitor from a current output of said current amplifier via a passive circuit such that when a voltage of charge on said controller power supply capacitor derived from said auxiliary power supply is less than said threshold operating voltage said power supply to said controller is provided by said charging of said controller power supply capacitor from said current output of said current amplifier and wherein said passive circuit substantially inhibits current flow off said controller power supply capacitor towards ground. 
   
     
     
         2 . A method as claimed in  claim 1 , wherein said auxiliary power supply comprises an auxiliary winding of said transformer. 
     
     
         3 . A method as claimed in  claim 1 , wherein said current amplifier comprises said power switch, wherein said bleeding of said current from said internal power supply comprises bleeding current though a resistor into a control terminal of said power switch, and wherein said output of said current amplifier comprises a connection to a switched terminal of said power switch. 
     
     
         4 . A method as claimed in  claim 3 , further comprising using a bipolar transistor for said power switch. 
     
     
         5 . A method as claimed in  claim 4 , wherein said power switch comprises a bipolar transistor emitter switched device, the method further comprising using said controller to switch said emitter connection of said bipolar transistor power switch to ground. 
     
     
         6 . A method as claimed in  claim 1 , wherein said controller comprises a controller IC, the method further comprising using an electrostatic discharge protection diode of said controller IC as an element of said passive circuit. 
     
     
         7 . A method as claimed in  claim 3 , wherein said resistor has a value of at least about 10MΩ or at least about 20MΩ, preferably at least about 40MΩ and most preferably at least about 50MΩ. 
     
     
         8 . A method as claimed in  claim 1 , wherein said controller is substantially inoperative to control said power switch until a threshold voltage of said power supply to said controller is reached, and wherein said controller draws less than 1 mA when substantially inoperative. 
     
     
         9 . A method of operating a switch mode power converter, comprising the method of  claim 1 , further comprising operating of said power supply with on-off switching of said power switch during a standby mode of said SMPC, and wherein, when said power switch is off, said current amplifier is substantially switched off. 
     
     
         10 . A method as claimed in  claim 9  when dependent on  claim 4 , further comprising passively clamping a voltage on said base terminal when said bipolar transistor power switch is off such that a voltage drop across said transistor is less than a turn-on threshold. 
     
     
         11 . A method as claimed in  claim 4 , further comprising limiting a voltage on said base connection of said bipolar transistor power switch when said power switch is off to maintain said bipolar transistor power switch off. 
     
     
         12 . A method as claimed in  claim 4 , further comprising clamping said base connection of said bipolar transistor power switch using a switching device such that said clamping of said base is selectively applied when said controller is operational, to control said switching device to maintain said bipolar transistor power switch off when controlled off by said controller despite said bleeding of said current from said internal power supply into said base connection. 
     
     
         13 . A method as claimed in  claim 12 , wherein said clamping said base connection of said bipolar transistor power switch using a switching device comprises coupling said switching device between said base connection and a regulated voltage and controlling said switching of said switching device to switch said switching device on to clamp said base when said controller is operating to switch said bipolar transistor power switch on and off. 
     
     
         14 . A method as claimed in  claim 1 , wherein said passive circuit comprises a rectifier. 
     
     
         15 . A method as claimed in  claim 8 , the method further comprising setting a start-up voltage for said internal power supply for operating switching of said controller by controlling a voltage on said base terminal of said bipolar transistor dependent on said voltage of said internal power supply. 
     
     
         16 . A method as claimed in  claim 2 , wherein said transformer is an auto transformer, and wherein said primary, secondary and auxiliary windings are defined by a single winding of said transformer. 
     
     
         17 . Bootstrap circuit for a switch mode power supply, the switch mode power supply for converting a voltage of an input power rail, the switch mode power supply comprising:
 a transformer having a primary winding coupled to receive power derived from said input power rail;   an auxiliary power supply;   an internal power supply;   a bipolar power switch;   a controller having a supply input and a control switch connected in series with the emitter of the bipolar power switch;   a reservoir capacitor configured to receive current from the auxiliary power supply and to determine voltage on the supply input of the controller,   
       the bootstrap circuit comprising:
 a current bleed impedance to bleed current from the internal power supply; 
 circuitry to deliver current from the internal power supply via the current bleed impedance to the base of the power switch such that the power switch is operable to amplify the current delivered from the internal power supply; 
 a passive circuit to provide the amplified current to the reservoir capacitor; and 
 the passive circuit further to substantially block reverse current flow from the supply input to the emitter of the power switch. 
 
     
     
         18 . The bootstrap circuit of  claim 17 , wherein said passive circuit comprises a rectifier. 
     
     
         19 . The bootstrap circuit of  claim 17 , wherein said switch mode power supply comprises an integrated circuit comprising said controller, and said passive circuit comprises an electrostatic discharge diode integral to said integrated circuit. 
     
     
         20 . The bootstrap circuit of  claim 17 , the bootstrap circuitry further comprising:
 a second rectifier and a second capacitor, said second rectifier configured to conduct charge derived from the base of the power switch towards the second capacitor.   
     
     
         21 . The bootstrap circuit of  claim 17 , the bootstrap circuitry further comprising:
 a second impedance connected to bleed current from the base of the power switch.   
     
     
         22 . The bootstrap circuit of  claim 21 , wherein a terminal of the current bleed impedance is connected to the base of the power switch via the second impedance. 
     
     
         23 . The bootstrap circuit of  claim 20 , further comprising a third rectifier configured to conduct current derived from said auxiliary power supply towards the second capacitor and to substantially block reverse current from the second capacitor to the auxiliary power supply. 
     
     
         24 . The bootstrap circuit of  claim 23 , further comprising an auxiliary capacitor configured to receive current from the auxiliary power supply and to determine voltage on an anode terminal of said third rectifier. 
     
     
         25 . The bootstrap circuit of  claim 23 , wherein said third rectifier comprises a switch element. 
     
     
         26 . The bootstrap circuit of  claim 25 , wherein:
 said switch element is configured to be switched on dependent on a voltage difference between said voltage on said auxiliary capacitor and voltage on said supply input to said controller; and   said switch element is configured to conduct current derived from a cathode terminal of said second rectifier when the switch element is switched on.   
     
     
         27 . The bootstrap circuit of  claim 26 , wherein:
 said voltage on said auxiliary capacitor is configured to reach a threshold voltage after said voltage on said supply input reaches a threshold voltage during start-up.   
     
     
         28 . The bootstrap circuit of  claim 17 , wherein the controller comprises circuitry to regulate the voltage on the supply input of the controller when the controller is operating. 
     
     
         29 . The bootstrap circuit of  claim 27 , wherein the controller comprises a sleep latch to activate the regulator when voltage on the supply input to the controller reaches or exceeds a threshold. 
     
     
         30 . The bootstrap circuit of  claim 17 , further comprising brownout protection circuitry, the controller for starting operating when the supply input reaches or exceeds a first threshold, the brownout protection circuitry to cause said voltage on said supply input to reach or exceed said first threshold when the voltage on the input power rail reaches or exceeds a second threshold,
 the brownout protection circuitry comprising:
 a third impedance configured to conduct current towards a reference voltage line; 
 the current bleed impedance connected to deliver current to the third impedance and to the base of the power switch; 
 the current bleed impedance and the third impedance connected such that the ratio of a value of the current bleed impedance to a value of the third impedance determines the second threshold. 
   
     
     
         31 . The bootstrap circuit of  claim 30 , the brownout protection circuitry further comprising:
 at least one rectifier connected in series with the third impedance to allow said current bled to said reference voltage line to flow via said third impedance such that the second threshold is further determined by forward voltage of the at least one rectifier.   
     
     
         32 . Controller for a switch mode voltage converter comprising a power switch, the controller comprising a control switch to switch current flow through said power switch on and off and the bootstrap circuit of  claim 17 . 
     
     
         33 . Switch mode voltage converter comprising the bootstrap circuit of  claim 17 . 
     
     
         34 . Bootstrap circuit for a switch mode power supply, the switch mode power supply for converting a voltage of an input power rail, the switch mode power supply comprising:
 a transformer having a primary winding coupled to receive power derived from said input power rail;   an auxiliary power supply;   an internal power supply;   a field effect power switch;   a controller having a supply input and a control switch connected in series with the source or drain of the field effect power switch; and   a reservoir capacitor configured to receive current from the auxiliary power supply and to determine voltage on the supply input of the controller, the bootstrap circuit comprising:   a current bleed impedance connected to the internal power supply;   circuitry to deliver current from the internal power supply via the current bleed impedance to the gate of the power switch such that the power switch is operable to amplify the current delivered from the internal power supply;   a passive circuit configured to provide the amplified current to the reservoir capacitor; and   the passive circuit further configured to substantially block reverse current flow from the supply input to the source of the power switch.